A number of circuits utilize phase detection circuits for measuring the phase difference of two signals. For example, frequency synthesizers based on phase locked loops are well known in the electronic arts. These circuits generate a signal which is a multiple of a reference frequency. The circuits utilize a feedback system in which the output signal's frequency is divided by X and fed to a phase detector which generates a signal indicative of the phase difference between the frequency divided output signal and the reference signal. This signal is then used to servo a voltage controlled oscillator until the output signal generated by the voltage controlled oscillator is X times the reference frequency.
Consider the case in which the two signals whose phase difference is to be determined differ substantially in frequency. The phase difference of these signals will increase linearly with time. Unfortunately, prior art phase detection circuits are not capable of measuring an arbitrarily large phase difference. The typical phase detection circuit generates a signal that is linearly related to the phase difference until the phase difference exceeds some predetermined value, usually about 360 degrees. As the phase increases past this value, the output signal returns to zero and starts to increase again. In effect, the output of these phase detectors is roughly proportional to the phase difference modulo 360 degrees.
The lack of linearity in the phase detection circuit has a deliterious effect on the settling time of a phase lock loop frequency synthesizer. To accommodate the range limitation associated with the output signal returning to zero at multiples of 360 degrees, an integrator must be included in the phase locked loop. The integrator and associated stabilizing components introduce time constants which increase the settling time of the loop when the output frequency is changed or the synthesizer is first turned on. In addition, the non-linearities in the phase detector also result in poor settling times when the output frequency differs greatly from the desired frequency, since the servo signal generated by a 370 degree phase difference is the same as that generated by a 10 degree phase difference.
Circuits that eliminate the "wrapping" of the phase difference signal at 360 degrees have been utilized to improve the performance of phase locked loops. These circuits generate a signal that is linearly related to the phase difference when the input signals are close in frequency. When the signals differ substantially in frequency, however, the output signal is a non-linear function of the frequency difference of the signals. Unfortunately, the non-linear relationship between the phase difference and the output signal present in these circuits also results in a significant increase in the settling time of a phase locked loop utilizing such circuits relative to the settling time that would be obtained if a linear phase detector had been utilized.
Broadly, it is the object of the present invention to provide an improved phase detection circuit.
It is a further object of the present invention to provide a phase detection circuit whose output is linearly related to the phase difference of the input signals even when the phase difference exceeds 360 degrees.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.